(Keynote) Monitoring and Preparation-Performance Trends of Co-Based Catalysts for Electrochemical Water Oxidation

Abstract

Photo- and electrochemical water splitting are direct routes to convert inexhaustible solar resources into storable chemical fuels. The development of noble metal-free catalysts is essential for forthcoming technological applications. To this end, we have launched systematic investigations into cobalt-based molecular oxoclusters and nanostructured oxides over the past years.[1, 2] In parallel, we explored in and ex situ monitoring methods for electrocatalysts, focusing on transition metal carbodiimides as structurally well-defined alternative to oxide-based materials. We investigated a series of transition metal carbodiimides MNCN (M = Co, Ni, Co0.9Ni0.1, Mn and Cu) for their possible interplay with surface oxide formation during electrochemical water oxidation using simultaneous operando Raman and X-ray absorption spectroscopy. Easily accessible and modifiable screen-printed electrodes were used as a proof of concept, providing the option to investigate catalyst loadings well below 1 mg. Operando oxide formation was only detected for CuNCN among the above series through changes in the XANES region at applied potential above 1.0 V vs. Ag/AgCl, while the other MNCN compounds remained stable.[3] Furthermore, we investigated the influence of materials processing on CoNCN-coated electrodes for electrochemical water oxidation with nanoscale depth profiling analysis using a tabletop XUV-laser ablation time of flight mass spectrometry setup. The wavelength was reduced to 46.9 nm for enhanced ablation characteristics and minimally invasive ablation pits. The results demonstrated that even on electrode spots that were deprived of the catalyst layer, surface confined residues of Co+ and N+ were clearly detectable by single XUV pulses. This method complemented the applied EDX and Raman mappings and offers new possibilities to track layer degradation and redeposition processes on electrocatalyst surfaces with high spatial resolution, thereby providing valuable insight for operational conditions.[4] Our recent studies on cobalt oxide catalysts as model systems are focused on the crucial influence of the preparative history on the chemical as well as photo- and electrocatalytic water oxidation performance. While hydrothermal processing is widely applied to obtain high surface nanomaterials, our recent in situ PXRD studies on hydrothermally formed Co3O4 water oxidation catalysts demonstrated that mechanistic insight is indispensable to select the optimal synthesis temperature window. Within the range between 185 and 200 °C, a significant change in the formation mechanism affected the activity and surface area of the Co3O4 nanoparticles. Currently, we have performed a comparative screening study of ten different synthetic approaches to Co3O4 with respect to their influence on the (photo)chemical vs. electrochemical water oxidation performance. The latter was found to be most independent of the synthetic methods, while the other assays showed a stronger dependence on the preparation pathway.[5] However, a parallel in-depth investigation into the synthetic parameters of convenient and fast microwave-hydrothermal pathways to Co3O4 nanoparticles demonstrated that the fine-tuning of processing parameters, such as temperature and precursor concentration, is essential to optimize their electrochemical performance. Comprehensive investigations to correlate synthetic conditions, growth mechanisms and emerging performance parameters for targeted catalyst optimization are now under way. [1] F. Song, R. Moré, M. Schilling, G. Smolentsev, N. Azzaroli, T. Fox, S. Luber, G. R. Patzke, J. Am. Chem. Soc. 2017 (139) 14198. [2] L. Reith, K. Lienau, D. S. Cook, R. Moré, R. I. Walton, G. R. Patzke, Chem. Eur. J. 2018 (24) 18424. [3] R. J. Müller, J. Lan, K. Lienau, R. Moré, C. A. Triana, M. Iannuzzi, G. R. Patzke, Dalton Trans. 2018 (47) 10759. [4] R. J. Müller, I. Kuznetsov, Y. Arbelo, M. Trottmann, C. S. Menoni, J. J. Rocca, G. R. Patzke, D. Bleiner, Anal.Chem. 2018 (90) 9234. [5] K. Lienau, L. Reith, C. A. Triana, G. R. Patzke, in preparation.